No, Singularity is not a Microkernel in a different language and yes it isn't totally new (but then what is?) you can find more information under >>Language-Based Protection<< in this article [1]. I don't think you could write an OS in F# (or at least it wouldn't be wise), since F# is basically an OCaml dialect and the OCaml people say they've created an Application language not a systems language.

Similarities to UNIX shouldn't surprise anybody either, if you look at OS books they are very UNIX heavy.